Method and apparatus for reading invisible symbol

ABSTRACT

An invisible symbol reading apparatus includes a heating unit for heating an invisible symbol formed on a sample and containing a material which emits infrared light when heated, a detecting unit for detecting infrared light emitted from the invisible symbol, and an arithmetic operation unit for binarizing a detection signal from the detecting unit. The arithmetic operation unit calculates a differential coefficient of the detection signal, that corresponds to a position on the sample. On the basis of upper and lower threshold values set for the differential coefficient, the arithmetic operation unit determines a maximum value of the differential coefficient in a region exceeding the upper threshold value and a minimum value of the differential coefficient in a region smaller than the lower threshold value. The arithmetic operation unit binarizes the detection signal by using the maximum or minimum value as a leading or trailing edge of a binary function.

This application is a continuation of U.S. application Ser. No.09/273,276, filed Mar. 22, 1999, now U.S. Pat. No. 6,168,081.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for readinginvisible symbols (linear barcodes and two-dimensional symbols) usablein mail service, distribution of articles whose appearances areimportant, management of documents and articles requiring secrecy, andprevention of forgery.

Barcodes or two-dimensional symbols representing information about amanufacturing country, manufacturer, and product items are printed orpasted as labels on many articles currently distributed on the market.When an operator uses an optical reader to read a barcode, acorresponding price is read out from a database. This greatly reducesthe working time compared to conventional work in which an operatorinputs a price into a register. Stock control is also made efficient byconstructing a POS system. Additionally, barcodes are effective toincrease the efficiency of express delivery.

On the other hand, barcodes cannot be attached to some articles, and itis better not to attach barcodes to some articles presently havingbarcodes. For example, a printed barcode spoils the appearance of abook. This problem of appearance can be solved if an invisible orstealth barcode can be attached to an article. A tag or barcode ispresently attached to the inside of a linen supply or clothing itemwhere this tag or barcode is difficult to see. However, the work ofdistribution can be rationalized if an invisible barcode can be attachedto the front surface of a packaged article.

In mail service, zip codes are read by an OCR to process a large amountof mails within a short time. However, this zip code reading istime-consuming and requires manual sorting of mails because read errorssometimes occur. Although barcodes may be used in mail service, visiblebarcodes cannot be printed on the surfaces of mails because the barcodescontaminate the mails. If information such as a zip code can be printedas an invisible barcode as in the above case, the time of sorting can begreatly reduced, and this allows rapid delivery of mails.

A barcode can contain much information in a narrow space. However, abarcode itself cannot unlimitedly shrink, so a fixed exclusive area isnecessary. This exclusive area is not negligible if the size of anarticle is small. However, an invisible barcode can be superposed onsome other printed information and hence does not require any exclusivearea.

As a method of preventing forgery, invisible barcodes can be combinedwith another forgery preventing method. This may improve the effect ofpreventing forgery. As described above, invisible barcodes can extendthe range of application of barcodes.

Two kinds of invisible barcode methods are presently possible: in onemethod a barcode is read by ultraviolet light, and in the other method abarcode is read by infrared light. In the method using ultravioletlight, a barcode is formed by using a fluorescent dye which does notabsorb visible light. This fluorescent dye is excited by ultravioletlight, and fluorescence whose wavelength is different from that of theexcitation light is detected. In the method using infrared light, abarcode is formed by using a metal complex which does not absorb visiblelight. This metal complex is excited by infrared light, and fluorescencewhose wavelength is different from that of the excitation light isdetected (“Stealth Barcodes”, Tsunemi Ooiwa, OplusE, No. 213, p. 83,1997).

Unfortunately, the ultraviolet light method has the following problem.That is, fluorescent dyes are often added to paper and cloth forbleaching purposes, and these fluorescent dyes also emit fluorescence.Since interaction with ultraviolet light is transition betweenelectronic states of molecules, fluorescence less depends upon theintrinsic nature of a substance. Therefore, it is highly likely thatreading of fluorescence emitted from an invisible barcode is interferedwith. Also, a fluorescent dye in the ultraviolet region readily causesphoto-deterioration, so it is highly possible that no predeterminedfluorescence intensity can be obtained after a long-time use or storage.For these reasons, the reading accuracy largely declines easily.

Additionally, both fluorescent dyes and metal complexes have problems intoxicity and waste disposal. That is, barcodes are brought into homestogether with commodities, and some barcodes remain existing in theliving environment for long time periods. Therefore, babies and littlechildren may lick these barcodes by mistake, or toxic low-concentrationexposure to barcodes may occur. When these possibilities are taken intoconsideration, it is necessary to select materials from compoundsalready found to be safe. Furthermore, when recent waste disposalregulations are taken into consideration, it is desirable to selectmaterials by taking account of even recycling and final disposal. Inthese respects, it is preferable to avoid the use of fluorescent dyesand metal complexes.

To prevent forgery, it is important that both a reader and an invisiblebarcode material be difficult to obtain. When an ultraviolet fluorescentdye is used, a light source for emitting ultraviolet light is readilyavailable. Fluorescence in the visible light region can, of course, bevisually checked. Fluorescence in the ultraviolet region is also easy tovisually check by inputting the fluorescence into another material.Additionally, fluorescence less depends upon the intrinsic nature of asubstance, so a substance which emits fluorescence similar to that of avisible barcode material is readily obtainable. On the other hand, inthe infrared light method using a metal complex, an LED for the nearinfrared region can be used as a light source, and fluorescence can bedetected by a CCD camera. Additionally, a similar fluorescent materialcan be easily obtained as in the case of a fluorescent material in theultraviolet region.

As described above, an invisible barcode presently has many problemsalthough it is expected as a technology meeting various needs.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus capable of reading invisible symbols with high readingaccuracy.

A method for reading an invisible symbol of the present inventioncomprises the steps of heating an invisible symbol formed on a sampleand containing a material which emits infrared light when heated,detecting infrared light emitted from the invisible symbol, calculatinga differential coefficient of a detection signal corresponding to aposition on the sample, determining, on the basis of upper and lowerthreshold values set for the differential coefficient, a maximum valueof the differential coefficient in a region exceeding the upperthreshold value and a minimum value of the differential coefficient in aregion smaller than the lower threshold value, and binarizing thedetection signal by using the maximum or minimum value as a leading ortrailing edge of a binary function.

In the method of the present invention, it is also possible to heat thesample and detect infrared light emitted from the invisible symbol in aprocess of cooling the sample.

An apparatus for reading an invisible symbol of the present inventioncomprises heating means for heating an invisible symbol formed on asample and containing a material which emits infrared light when heated,detecting means for detecting infrared light emitted from the invisiblesymbol, and an arithmetic operation unit for binarizing a detectionsignal from the detecting means. The arithmetic operation unitcalculates a differential coefficient of a detection signalcorresponding to a position on the sample, determines, on the basis ofupper and lower threshold values set for the differential coefficient, amaximum value of the differential coefficient in a region exceeding theupper threshold value and a minimum value of the differentialcoefficient in a region smaller than the lower threshold value, andbinarizes the maximum or minimum value as a leading or trailing edge ofa binary function.

The apparatus of the present invention can further comprise means formoving the sample from a heating position of the heating means to adetection position of the detecting means, and control means for turningoff the heating means heating the sample before detection by thedetecting means. When these means are provided, infrared light emittedfrom the invisible symbol can be detected in a process of cooling thesample.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a block diagram showing an invisible symbol reading apparatusof Example 1;

FIG. 2 is a graph showing the intensity of infrared light emitted froman invisible symbol measured in Example 1;

FIG. 3 is a graph for explaining a method of determining peak valuesfrom differential data of measured data in Example 1;

FIG. 4 is a graph showing differential data and a binary function inExample 1;

FIG. 5 is a graph showing a binary function and data obtained byoptimizing the widths of bars and spaces;

FIG. 6 is a block diagram showing an invisible symbol reading apparatusof Example 2;

FIG. 7A is a perspective view of a stage and a target used in theinvisible symbol reading apparatus of Example 2, FIG. 7B is aperspective view of the target, and FIG. 7C is a sectional view of thestage and the target;

FIG. 8A is a plan view showing an invisible symbol reading apparatus ofExample 5 and FIG. 8B is a front view of the apparatus;

FIG. 9 is a view showing the construction of an invisible symbol readingapparatus of Example 6;

FIG. 10 is a perspective view showing the positional relationshipbetween a tubular halogen lamp with reflector and a stage top plate inthe invisible symbol reading apparatus;

FIG. 11 is a sectional view of the tubular halogen lamp with reflectorof the invisible symbol reading apparatus of Example 6;

FIG. 12 is a graph showing infrared light intensity emitted from ameasured barcode in Example 6;

FIG. 13 is a view showing the operation of a thermal head in aninvisible symbol reading apparatus of Example 7;

FIG. 14 is a plan view showing a barcode information portion and itsperipheral portion;

FIG. 15 is a block diagram showing an invisible symbol reading apparatusof Example 10;

FIGS. 16A and 16B are graphs showing infrared emission signals obtainedby different spatial resolutions;

FIG. 17 is a perspective view showing an arrangement of a detectingoptical system, heating means, and conveyor means of an invisible symbolreading apparatus of Example 12; and

FIG. 18 is a view showing another arrangement of the detecting operationsystem, heating means, and conveyor means of the invisible symbolreading apparatus of Example 12.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in more detail below.

In the present invention, symbols mean linear (one-dimensional) barcodesand two-dimensional symbols. Reading of a barcode as a representativesymbol will be principally described below.

The principle of the present invention is to use infrared light emittedfrom an invisible barcode as reading light. Every molecule hasabsorption in the infrared region as interaction with its molecularvibration. Therefore, every molecule absorbs infrared light having awavelength intrinsic to its molecular structure and state and emitsinfrared light. That is, the wavelength of infrared light as a result ofinteraction changes in accordance with the molecular structure or stateof a molecule. Accordingly, the influence of an interfering substancecan be suppressed by using a material not used in a sample (an articleto which a barcode is attached) as the material of the barcode.Additionally, a material containing neither metals nor halogens andfound to be safe both when in use and wasted can be chosen as thematerial of a barcode. When a barcode is to be read with infrared light,the energy per photon is smaller than that of ultraviolet light orvisible light, so the operation is readily influenced by ambient heat.By contrast, the present invention can effectively remove ambientthermal noise.

That is, an invisible symbol reading method of the present inventioncomprises the steps of heating an invisible symbol formed on a sampleand containing a material which emits infrared light when heated,detecting infrared light emitted from the invisible symbol, calculatinga differential coefficient of a detection signal corresponding to aposition on the sample, determining, on the basis of upper and lowerthreshold values set for the differential coefficient, a maximum valueof the differential coefficient in a region exceeding the upperthreshold value and a minimum value of the differential coefficient in aregion smaller than the lower threshold value, and binarizing thedetection signal by using the maximum or minimum value as a leading ortrailing edge of a binary function.

In the present invention, an invisible symbol formed on a sample andcontaining a material which emits infrared light when heated is heated,and infrared light emitted from the invisible symbol as a result ofheating is detected.

If the heating means exists near the sample while a signal is detected,heat from this heating means is detected as a noise, and this decreasesthe sensitivity. Also, when a signal is detected while the sample isheated, heat absorbed as infrared light by the barcode diffuses to theunderlying substrate. This decreases the signal contrast between thebarcode and the substrate. In the present invention, therefore, it isalso possible to heat a sample on which an invisible barcode is formedand detect infrared light emitted from the barcode in the course ofcooling the sample.

More specifically, heating means is installed apart from detectingmeans, and a sample is moved from the heating position of the heatingmeans to the detection position of the detecting means. Alternatively,the heating means heating a sample is turned off before detection by thedetecting means. When a method like this is used to detect infraredlight emitted from a barcode during the course of sample cooling,thermal noise from the heating means can be reduced, and any decrease ofcontrast between the barcode and the substrate can be prevented.

Next, the detection signal obtained by the detecting means is binarized.This detection signal of a barcode is a curve corresponding to bars(black portions in common barcodes) and spaces. If there is no influenceof thermal noise, it is possible to set an appropriate threshold value,regard portions larger and smaller than this threshold value to be barsand spaces, respectively, and determine points intersecting thethreshold value as the start and end positions of each bar. However,even when thermal noise from the heating means is reduced as describedabove, ambient thermal noise is superposed on the detection signal ofinfrared light, so the whole detection signal often fluctuates. Hence,this simple method cannot binarize the signal.

In the present invention, the detection signal is differentiated, andmaximum and minimum values of the differential coefficient are used todetermine the start and end positions of a bar. More specifically, twothreshold values, i.e., upper and lower threshold values are set with apredetermined width between them with respect to the differentialcoefficient of the detection signal. A maximum value of the differentialcoefficient in a region exceeding the upper threshold value and aminimum value of the differential coefficient in a region smaller thanthe lower threshold value are determined. The detection signal isbinarized by using these peak values as the leading or trailing edge ofa binary function. Note that the detection signal is desirably smoothedto reduce the influence of thermal noise before being differentiated. Itis further desirable to smooth the slope in calculating the differentialcoefficient.

In actually used barcodes, the widths of bars (or spaces) are integralmultiples of a basic width. However, the binary function obtained by theabove method often shifts from an integral multiple of the basic widthowing to the influence of noise or smoothing. Therefore, it is desirableto perform data processing of correcting the obtained binary functioninto an integral multiple of the basic width. When the data thusobtained is decoded in the same manner as for a common barcode,information recorded in the invisible barcode can be read.

The reading accuracy can be increased by using various rules applied tobarcodes. For example, assuming a bar with the basic width is “1” and aspace with the basic width is “0”, a JAN code of eight characters isrepresented in accordance with the following rule. That is, a JAN codeof eight characters is composed of left guard bars “101”, four datacharacters on the left side, center bars “01010”, three data characterson the right side, one modular check character, and right guard bars“101”. One character (number) is expressed by forming a module seventimes as large as the basic width by using two bars and two spaces. Theboundary between characters coincides with a change from a bar to aspace. The modular check character is used to check for a read error andtakes a value calculated on the basis of the data characters.

An apparatus of the present invention can increase the reading accuracyby checking whether the result of binarization meets the aforementionedrules. The apparatus can also correct the result of binarization toeliminate contradiction. Details of the data processing method will bedescribed later in examples of the present invention.

U.S. Pat. No. 5,294,198 discloses a semiconductor device evaluationapparatus for obtaining surface information of a sample by detectinginfrared light emitted by heating. This apparatus observes infraredlight emitted from different portions on the surface of a device andevaluates whether the device is normal or abnormal on the basis of theinfrared emission (temperature). In this evaluation, the apparatuschecks whether the deviation of each measured value from a referencevalue exceeds a predetermined value. However, an invisible symbol suchas an invisible barcode as an object of the present invention cannot beread by simply-comparing the measured value with the reference value asin this apparatus.

Jpn. Pat. Appln. KOKAI Publication No. 7-282184 describes an apparatusfor sensing infrared light emitted by heating to discriminate aninvisible symbol for examining the genuineness of an article. In thisapparatus, a heat roller heats an invisible symbol formed on the surfaceof a card by using a heat-absorbing substance, and an array sensorsenses the thermal emission of light. The apparatus examines thegenuineness of the card on the basis of matching between the magneticinformation and the light-emitting invisible symbol. Unfortunately, anobject of detection by this prior art is a genuineness examination marklarger than general barcodes. To read symbols such as barcodes as in thepresent invention, an optical system for detecting emission of infraredlight with spatial resolution meeting the minimum width of a barcode isimportant. However, Jpn. Pat. Appln. KOKAI Publication No. 7-282184 doesnot describe any optical system for guiding light emission to aninfrared sensor such as an array sensor. Hence, the apparatus cannot beapplied to reading of barcodes.

Materials used in the present invention and components of a readingapparatus will be described in more detail below.

In the present invention, a compound having an infrared absorptionwavelength far apart from that of a sample (substrate) is used as thematerial of an invisible barcode. Since infrared light is absorbed byatmospheric moisture or carbon dioxide depending on the wavelength, theintensity of a detection signal can largely vary in some cases. Thedetection signal is also readily influenced by water or contamination onthe surface of an invisible barcode. Accordingly, it is important toeliminate these influences. It is, therefore, desirable to use acompound having a cyano. (CN) group as the material of an invisiblebarcode, and the use of a polymer containing a cyano group is moredesirable. This material is used in the form of ink-jet printer ink,thermal transfer ink ribbon, electrophotographic toner, or fiber to forman invisible barcode on a sample made of paper, polymer, cloth, or thelike. For example, when an invisible barcode made from a polymercontaining a cyano group is heated, the barcode emits infrared lightwith a wavelength of about 4.5 μm.

To obtain emission of infrared light from an invisible barcode byexciting molecular vibration of the barcode, a barcode containing amaterial which absorbs infrared light is heated. The heating means isdesirably a contact type heater such as a thermal head, thermal bar, orhot stamp; a warm air heater; or a halogen lamp which emits infraredlight in a broad wavelength range. To excite the molecular state of abarcode, it is also possible to irradiate light having a specificwavelength. However, this method is undesirable in terms of efficiencybecause light except for the specific wavelength is cut.

In the present invention, the heating means is preferably capable ofheating the entire area of a barcode information portion at once. When ahalogen lamp is used as this heating means, for example, a tubularhalogen lamp is selected, and a bifocal reflector having an ellipticsection is installed around the lamp. The lamp is positioned at onefocal point of the reflector, and a sample is positioned at the otherfocal point. The reflector linearly condenses infrared light from thehalogen lamp onto the sample and heats the sample with this lightwithout contacting the sample. When the output of the halogen lamp isabout 1 kW, however, a sample may be overheated to 100° C. or more in afew seconds if the sample is positioned at the focal point of thereflector. To prevent this, it is preferable to install means capable ofadjusting the vertical position of the halogen lamp and place a samplein a position shifted a few mm from the focal point of the reflector. Itis also preferable to heat a sample to a fixed temperature bydetermining the heating time and current value by measuring thetemperature of the sample by a radiation thermometer and install asafety device for protecting the sample from overheating.

For a sample unsuited to being heated to a high temperature, e.g., acard in which magnetic information is written, it is necessary to heatonly the barcode portion and hold the card main body at a lowtemperature. To this end, a heater is desirably brought into contactwith the barcode printed surface to heat it.

When any of these heating means is used to heat a barcode, excessiveheating of the surroundings causes thermal noise. Therefore, it ispreferable to heat a barcode by adjusting not only the ultimatetemperature but also the way the temperature is changed in accordancewith each sample.

From the viewpoint of the read accuracy, the appropriate heatingtemperature of a sample has a certain relation to the sensitivity of adetector. That is, when the sensitivity of a detector is high, it isdesirable to minimize the heating temperature of a sample to reduceunnecessary thermal noise. For example, when an MCT detector is used aswill be described later, the heating temperature of a sample ispreferably 50 to 100° C., and more preferably 70 to 80° C.

When a heated sample is moved from the heating position to the detectionposition so as to be put in the cooling process as described earlier,the moving direction can be either perpendicular or parallel to the scandirection of a barcode. Examples of the moving means are a steppingmotor and conveyor rollers. It is also possible to use a hot stagehaving a built-in heating means such as a bar heater and turn off theheating means before a heated sample is subjected to detection bydetecting means. This hot stage preferably has a mechanism capable ofradiating heat and cooling. In order to make a sample brought intocontact with the hot stage uniformly in plane, it is desirable to use atarget described below.

Infrared light emitted from a barcode is invisible, so it is difficultto align the optical axis of a detecting optical system with a barcodeinformation portion on a sample. To allow easy alignment, therefore, itis also possible to use a target for alignment and align a barcoderegion with this target. For example, a target obtained by forming across-shaped mark matching the optical axis of the optical system on atransparent film is used. Alternatively, a target having marks in threeto four portions of a frame is used to align the intersection ofextension lines of the marks with the optical axis of the opticalsystem. Visible light can also be irradiated as guide light to attainalignment with the optical axis of the optical system. As a light sourceof this visible light, a low-output diode laser with a wavelength ofabout 650 nm can be used.

To detect infrared light emitted from a sample having a barcode on it inaccordance with the position on the sample, the optical axis of theoptical system and the sample are moved relative to each other. Twomethods are possible for this purpose: one is a method of scanning theoptical axis on the sample by rotating an optical element, and the otheris a method of conveying the sample by a conveyor mechanism. The anglethe optical axis of the optical system makes with a bar of a barcodesometimes changes in accordance with the position, and this may changethe apparent bar width to be read. To prevent this, it is desirable toeliminate the dependence of the bar width on the position by positioninga sample at the focal point on the optical axis of the optical systemand conveying the sample while a fixed angle is held between the opticalaxis and the sample. It is particularly desirable that the optical axisof the optical system and the sample surface be perpendicular to eachother. Note that if the apparent bar width changes in accordance withthe position, data correction is performed. To convey a sample by theconveyor mechanism, the way the sample is moved is adjusted inaccordance with a barcode. Since the minimum bar width (basic width) ofa common barcode is about 250 μm, the conveyance step is preferably 100μm or less, and more preferably about 10 μm. When a detector composed ofa satisfactorily large number of elements, e.g., an FPA (Focal PlaneArray) is used, pieces of information concerning different portions of asample can be obtained at once. This eliminates the need to convey thesample to read signals.

A mirror or a lens is used as an optical element for focusing andguiding infrared emission from a sample to the detector. When the objectto be detected is a common barcode whose minimum width is about 250 μm,a proper optical element is chosen in accordance with the size of anelement of the detector. When the size of each element constructing thedetector is 100 μm or less, a barcode can be detected withsatisfactorily high spatial resolution, so the optical element can beeither a mirror or a lens. If the size of each element constructing thedetector is larger than 100 μm, it is necessary to enlarge an imagebefore image formation. Hence, the use of a lens or a combined mirror isdesirable. If this is the case, a Cassegrain lens used in a microscopicoptical system is desirable, and a lens with a large work length is moredesirable. The material of the lens is so selected as to meet thewavelength of infrared light to be detected. If visible light is used asguide light, a material which transmits both visible light and infraredlight is chosen as the lens material. An optical stop is desirablyinserted on the optical axis of the optical system to improve thequality of light reaching the detector. If light beams having adifferent wave length from each other are incident to a refractiveoptical element, they pass different optical paths due to differencebetween focal lengths. Therefore, the use of the refractive opticalelement and the optical stop is advantageous because infrared light witha specific wavelength can be detected.

A method of selecting a wavelength can be used to reduce ambient thermalnoise and detect infrared emission from a sample. To select awavelength, a grating or a filter can be used. A grating can extract awavelength in a narrow range, but the utilization (throughput) of lightis low, and the apparatus is enlarged. A filter is obtained byperforming appropriate optical processing for a substrate suited to awavelength to be used and hence can be used easily. High-pass, low-pass,and bandpass filters can be selectively used. Infrared emission from abarcode shows peaks centering around a wavelength due to molecularvibration, whereas ambient thermal noise is independent of wavelength.By using a bandpass filter having a central wavelength corresponding tothe wavelength of infrared emission from a barcode, it is possible toeffectively remove thermal noise and selectively guide the infraredemission from the barcode to the detector. For example, when a barcodeis formed by using polyacrylonitrile containing a cyano group, thewavelength of infrared emission is around 4.5 μm, so a bandpass filterwhich transmits this wavelength is used. Bandpass filters are classifiedinto a wide-bandpass filter (10% or more of the central wavelength) anda narrow-bandpass filter (2% to 10% of the central wavelength) inaccordance with the band width. The narrower the band width, the higherthe efficiency of thermal noise removal, but the smaller the transmittedlight intensity. Hence, it is desirable to use a bandpass filter havingan appropriate band width in accordance with the sensitivity and S/Nratio of the detector.

As another method of reducing ambient thermal noise and detectinginfrared emission from a sample, a method of optically modulatinginfrared emission and detecting the phase by using a lock-in amplifieris also effective. To optically modulate infrared emission, it ispossible to use a method using an optical chopper, a tuning-fork choppera polygon mirror or a galvano-mirror or a method which performspolarization modulation by additionally using a polarizing element. Toavoid distortion of signals and reduction of the light intensity, theuse of an optical chopper or a tuning-fork chopper is desirable. Themodulation frequency is desirably 1 Hz to 100 kHz, and more desirably 10Hz to 10 kHz.

To read a barcode, infrared emission can also be subjected to ACcoupling amplification. When a sample having a barcode is conveyed at arate at which the basic width of bars can be scanned in a time notexceeding the reciprocal of the cutoff frequency of an AC couplingamplifier (e.g., a time of 200 ms or less if the cutoff frequency is 5Hz), infrared emission can be amplified without being influenced byambient thermal noise. If the intensity of a signal whose phase is to bedetected by a lock-in amplifier is low, a preamplifier is preferablyinstalled before the lock-in amplifier to amplify the signal intensityby 10 to 100 times.

When a barcode is read by spatial resolution equivalent to the basicwidth of the barcode, the read time can be reduced by increasing theconveyance rate of a sample, but the apparatus function is superposed(convoluted) on the amplitude of a signal. If this is the case, it isdesirable to use a filter as a function of the signal frequency on thesignal to remove (deconvolute) the apparatus function and correct theamplitude and then binarize the signal.

As an infrared detector, a high-sensitivity detector having asensitivity region meeting infrared emission from a barcode is used.When a barcode is formed by a polymer containing a cyano group, it isdesirable to use a detector having an element made from MCT (MercuryCadmium Tellurium), InSb (indium antimony), or PtSi (platinum silicide),each of which has high sensitivity near 4.5 μm. Any of these detectorsis a quantum detector which detects infrared emission as light, so thedetector is cooled to a low temperature to reduce thermal noise from thedetecting element itself. The cooling means can be any of cooling usingliquid nitrogen, electronic cooling using a Peltier element, Stirlingcooling using a compressor, a pulse-tube cooling, and J-T(Joule-Thomson) cooling using adiabatic expansion. To perform cryogeniccooling, the use of liquid nitrogen, Stirling cooling or a pulse-tubecooling is desirable. Note that a detector such as a bolometer whichdetects infrared emission as heat requires no cooling and hence can besuitably used provided that the system generates intense signals or thedetector has high sensitivity.

To distinguish between a signal from a barcode and thermal noise, it ispreferable to regard the signal level of the underlying substrate as thesignal level of background and correct a measured detection signal onthe basis of this signal level. It is also possible to correct ameasured detection signal by regarding the average signal level in abroad range including both an information portion and a peripheralportion (substrate) as the signal level of background. In this method,however, the background signal level is estimated to be higher than theactual level, so the contrast between the information portion and theperipheral portion lowers when correction is performed. Therefore, it isdesirable to obtain the signal level of only the peripheral portion. Tothis end, a signal from the peripheral portion can be detected in adifferent step from the step of detecting a signal from the barcodeinformation portion.

Also, the signal level of the peripheral portion is preferablyelectrically corrected by AC coupling amplification as follows. That is,the detection position is so moved as to alternately reciprocate over aninformation portion and a peripheral portion of a barcode across theedge of the barcode in a time not exceeding the reciprocal of the cutofffrequency of an AC coupling amplifier. The moving range is about 1 toten-odd times the spot diameter from the edge of the barcode informationportion. A signal change caused by this reciprocal motion can beseparated, in accordance with the frequency, from a signal changeresulting from conveyance of the barcode in the scan direction.Consequently, the detection position is preferably reciprocated at arate 10 times the conveyance rate or more. This method can remove evenslight thermal noise by correction using the signal level of theperipheral portion.

EXAMPLES

Examples of the present invention will be described below.

Example 1

An acrylonitrile (25%)-styrene (75%) copolymer (AS resin) was used asthe material of an invisible barcode. This resin was pulverized to havean average size of 11 μm to prepare toner not containing pigments. Thistoner was used as toner of a laser beam printer to form an invisiblelinear barcode on plain paper. The formed barcode corresponds to anenlarged size with a basic width of 3 mm obtained by enlarging a JANcode with a basic width of 300 μm printed on an existing articleselected at random.

FIG. 1 is a block diagram showing an invisible symbol reading apparatusused in this example. A sample 1 on which the invisible barcode isprinted is held on a pulse stage 11. This pulse stage 11 moves inaccordance with a signal from a stage controller 12. The sample 1 isheated by warm air blown from a warm air heater 13. This heating excitesmolecular vibration of a cyano group in the invisible barcode, andinfrared emission occurs near 4.5 μm accordingly.

This infrared emission is reflected by an elliptic mirror 15 through anoptical chopper 14 and detected by an MCT detector 17 through a bandpassinfrared filter 16. The elliptic mirror 15 has a focal length of 100 mmand forms an image without changing the magnification. The MCT detector17 has a highest-sensitivity wavelength of 4.5 μm, and itslight-receiving surface is composed of square elements of 1 mm side.This MCT detector 17 is electronically cooled by a Peltier element andused at −60° C. A bias power supply 18 supplies power to the MCTdetector 17. The MCT detector 17 converts a change in its electricalresistance caused by infrared light into a voltage and thereby generatesa detection signal.

A preamplifier 19 amplifies the output from the MCT detector 17 by 100times, and a lock-in amplifier 20 detects and amplifies the in-phasesignal. A digital sampling oscilloscope (not shown) triggered by anoutput from the optical chopper displays the waveform of the detectionsignal. The detection signal is input to an A/D conversion board of apersonal computer 21 and subjected to data processing (to be describedlater). A decoder 23 decodes the processed signal.

The operation was actually performed as follows. The sample 1 was heldon the pulse stage 11 by a plate-like magnet and so adjusted that theinvisible symbol region on the sample was positioned at the focal pointof the optical system. The warm air heater 13 was so installed as toblow warm air against the sample 1. The position of blow of warm air wasset upstream of the focal point (the position moved closer to the focalpoint when the stage moved), and a signal was detected. As aconsequence, the signal intensity increased when the signal was measuredby heating a position about 3 mm from the detection position by blowingwarm air. Infrared emission at the focal point was detected while thepulse stage 11 was moved at a fixed rate by the signal from the stagecontroller 12. The movement of the pulse stage 11 was monitored by anoptical sensor and measured by taking margins before and after theinvisible barcode. Data of the detection signal was input as a file tothe personal computer 21.

FIG. 2 shows a detection signal when the sample was heated to 70° C. Inthis detection signal, peaks and valleys are formed in accordance withbars and spaces, and the widths of the peaks (valleys) change inaccordance with the widths of the bars (spaces). However, ambientthermal noise is superposed on the detection signal, so the signal wavesas a whole. This makes it impossible to apply the method which sets anappropriate threshold value, regards portions larger and smaller thanthis threshold value as bars and spaces, respectively, and determineintersections to the threshold value as the start and end positions ofeach bar.

Hence, the measured data was processed as follows. First, the data wassmoothed by using the moving average method to remove high-frequencynoise. This high-frequency noise was removed when the rating ofsmoothing was 41 or more, for example, as a basic width corresponds withabout 100. Next, the detection signal was differentiated forbinarization. To remove the influence of thermal noise, the slopes atsurrounding points were smoothed to calculate a differential coefficienty′.

FIG. 3 is an enlarged view showing a partial change in the differentialcoefficient y′ (ordinate) corresponding to the position (distance on theabscissa) on the sample. As shown in FIG. 3, two adequate thresholdvalues were set for y′. An extreme value (maximum value) of y′ in aregion exceeding the upper threshold value and an extreme value (minimumvalue) of y′ in a region smaller than the lower threshold value arecandidates of the start and end positions of a bar (space). Peaksbetween the two threshold values were removed by regarding them asnoise. Note that changing the threshold values by a magnitude of 5% orless of the amplitude (difference between the maximum and minimumvalues) had no influence on the results.

Bars and spaces alternately appear in an actual barcode, so the peaks ofy′ are also supposed to alternately appear above and below the thresholdvalues. However, two peaks (peaks in ranges B and C in FIG. 3) canappear in a region smaller than the lower threshold value owing to theinfluence of thermal noise. If this is the case, real peaks aredetermined following a procedure below. First, a minimum value of y′appearing for the first time in a region smaller than the lowerthreshold value is regarded as a provisional minimum peak. As in a rangeA, if y′ does not decrease after the provisional minimum peak and amaximum peak appears in a region exceeding the upper threshold value,the provisional minimum peak is considered to be a real minimum peak. Onthe other hand, as in the ranges B and C, if y′ again decreases afterthe provisional minimum peak and before exceeding the upper thresholdvalue and a minimum peak appears in the region smaller than the lowerthreshold value, these two minimum values in the ranges B and C arecompared, and a smaller one is considered to be a real minimum peak. Areal maximum peak is determined following the same procedure. Addressesx(i) of the leading and trailing edges of a binary functioncorresponding to a peak value y′ (i) thus determined are the start andend positions of an actually measured bar. FIG. 4 collectively showsdata of the differential coefficient y′ and a binary function whichrises and falls in the start and end positions, respectively, of a bar.

The width of a bar or a space is supposed to be obtained when thedifference between two continuous addresses x(i+1) and x(i) iscalculated. However, in this calculation the width of a bar or a spacetended to be smaller than an integral multiple of the basic width underthe influence of smoothing. In contrast, the total width of an adjacentbar and space was an integral multiple of the sum of the basic widths ofthe two. A detailed calculation procedure is as follows.

The difference between start addresses (or end addresses) x(i+2) andx(i) of adjacent bars is calculated as a width X(i). This X(i)corresponds to the total width of an adjacent bar and space. A pluralityof X(i)'s are sorted and arranged in ascending order. A half value ofthe average of two smallest X's is calculated as an initial value of abasic width W. A value 2.5 times this W is used as a threshold value,and X's assumed to have a width W₂ which is twice the basic width areselected from X's equal to or smaller than the threshold value. Theaverage value of these X's is calculated as new W. This new W and theinitial value are compared, and the process is; repeated until the twovalues are equal. W finally obtained by this operation is regarded as asecond initial value of W. A value 3.5 times this second W is used as athreshold value, and X's assumed to have a width W₃ which is three timesthe basic width are selected from X's equal to or smaller than thethreshold value. The average value of these X's is calculated as new W.This new W and the second initial value are compared, and the process isrepeated until the two values are equal. W finally obtained by thisoperation is regarded as a third initial value of W. In the same manneras above, a threshold value is calculated by using W finally obtained inthe immediately preceding calculation as a new initial value. Similarcalculations are repeatedly performed for W₄ and W₅ to obtain W as afifth initial value. The value of each X(i) is divided by the fifthinitial value W and rounded to obtain an integer. W is again calculatedon the basis of X's from X assumed to have the width W₂ to X assumed tohave the width W₅. This new W and the fifth initial value are compared,and the process is repeated until the two values are equal. In thismanner a final basic width W is obtained.

The positions of bars and spaces are determined in units of the obtainedbasic width W. To align the start position of the barcode whilecorrecting any offset caused by noise, let the entire offset be d andthe corrected value of W be ω. While ω is changed in units of 0.001 Wfrom 0.99 W to 1.01 W and d is changed in units of 0.01ω from −0.5ω to0.5ω for certain ω, summation Σδ of differences δ between a (bar startor end address) and an (integral multiple of ω) is calculated as per${\sum\delta} = {\sum\limits_{i = 0}^{n}\quad {{{x(i)} - {{{INT}\left( {\frac{{x(i)} - d}{\omega} + 0.5} \right)} \times \omega} + d}}}$${y(i)} = {{INT}\left( {\frac{{x(i)} - d}{\omega} + 0.5} \right)}$

Σδ for different combinations of ω and d are compared, and a combinationof ω and d by which Σδ is a minimum is determined.

The bar start or end address x(i) is corrected by the offset d andrepresented by an integral multiple y(i) of ω (an integer is obtained byrounding). A bar or space width Y(i) is calculated from the differencebetween the integral addresses y(i). FIG. 5 shows data obtained byoptimizing the final bar or space width thus obtained and a binaryfunction. This optimized data was decoded by the decoder 23.Consequently, the decoded data matched the result of decoding ofenlarged data of the original data printed on the existing article.

Example 2

Toner made from the same AS resin as used in Example 1 was used as tonerof a laser beam printer to form an invisible barcode on plain paper. Theformed barcode corresponds to a standard-size barcode with a basic widthof 300 μm printed on an existing article selected at random.

FIG. 6 is a block diagram showing an invisible symbol reading apparatusused in this example. A holder 32 having a built-in bar heater 31 holdsa sample 1 on which the invisible barcode is printed on a pulse stage11. A signal from a built-in thermocouple (not shown) of the holder 32is input to a temperature controller 33 to adjust the current to besupplied to the bar heater 31, thereby heating the sample 1 to apredetermined temperature. A target 34 is used to align the optical axisof an optical system and a barcode formation region of the sample 1. Asshown in FIG. 7A or 7B, this target 34 is a frame-like member, and threeor four marks 34 a are formed on it. As shown in FIGS. 7A to 7C, theintersection of extension lines of the marks 34 a is aligned with theoptical axis of the optical system. The target 34 functions as a keepplate to hold down the sample 1 on the holder 32.

An MCT detector 17 detects infrared emission from the invisible barcodethrough an optical chopper 14, a Cassegrain lens 35, and a bandpassinfrared filter 16. An optical stop (not shown) is installed on theoptical axis. The Cassegrain lens 35 has a focal length of 13 mm andforms an enlarged image of ×15. The MCT detector 17 has ahighest-sensitivity wavelength of 4.5 μm, and its light-receivingsurface is composed of square elements of 1 mm side. This MCT detector17 is electronically cooled by liquid nitrogen and used at −200° C. Abias power supply 18 supplies power to the MCT detector 17.

As in Example 1, a preamplifier 19 amplifies the output from the MCTdetector 17 by 100 times, and a lock-in amplifier 20 detects andamplifies the phase of the signal. A digital sampling oscilloscope (notshown) triggered by an output from the optical chopper displays thewaveform of the detection signal. The detection signal is input to anA/D conversion board 22 of a personal computer 21 and subjected to dataprocessing (to be described later). A decoder 23 decodes the processedsignal.

The operation was actually performed as follows. The sample 1 was heldon the pulse stage 11 by a plate-like magnet and so adjusted that theinvisible barcode region on the sample was positioned at the focal pointof the optical system. The sample 1 was heated to 70° C. under thecontrol of the temperature controller 33. Infrared emission at the focalpoint was detected while the pulse stage 11 was moved at a fixed rate bythe signal from the stage controller 12. The start and end positions ofthe barcode were measured by taking margins for these positions by usinglimit switch signals from the pulse stage 11. Data of the detectionsignal was input as a file to the personal computer 21.

Following the same procedures as in Example 1, the measured data wassmoothed and binarized by differentiation, and the bar and space widthswere optimized. This optimized data was decoded by the decoder 23.Consequently, the decoded data matched the result of decoding of theoriginal data printed on the existing article.

Example 3

A polyacrylonitrile powder was dispersed in a 5 wt % aqueous polyvinylalcohol solution at a ratio of 2 wt % with respect to polyvinyl alcohol.The resultant dispersion was used as ink of an ink jet printer to forman invisible barcode on plain paper. The formed barcode corresponds tothe same standard-size barcode with a basic width of 300 μm as inExample 2.

A reading apparatus shown in FIG. 6 was used to heat a sample 1 to 70°C. while a pulse stage 11 was moved at a fixed rate. Infrared emissionat a focal point was detected and input to a personal computer 21. Thestart and end positions of the barcode were measured by taking marginsfor these positions by using limit switch signals from the pulse stage11. Following the same procedures as in Example 1, the measured data wassmoothed and binarized by differentiation, and the bar and space widthswere optimized. This optimized data was decoded by a decoder 23, thedecoded data matched the result of decoding of the original data.

Example 4

An acrylonitrile-styrene copolymer (AS resin) was dissolved in toluene,and the solution was mixed with wax. A substrate was coated with theresultant material and dried to form a heat-sensitive ink ribbon. Thisheat-sensitive ink ribbon was used to form an invisible barcode on plainpaper. The formed barcode corresponds to the same standard-size barcodewith a basic width of 300 μm as in Example 2.

A reading apparatus shown in FIG. 6 was used to heat a sample 1 to 70°C. while a pulse stage 11 was moved at a fixed rate. Infrared emissionat a focal point was detected and input to a personal computer 21. Thestart and end positions of the barcode were measured by taking marginsfor these positions by using limit switch signals from the pulse stage11. Following the same procedures as in Example 1, the measured data wassmoothed and binarized by differentiation, and the bar and space widthswere optimized. This optimized data was decoded by a decoder 23, and thedecoded data matched the result of decoding of the original data.

Example 5

FIGS. 8A and 8B show details of the main parts of the reading apparatusaccording to the present invention. FIG. 8A is a plan view, and FIG. 8Bis a front view. A lens barrel 101, an MCT detector 102, and a liquidnitrogen tank 102A for cooling vertically extend above the samplesurface. A reflecting objective lens 103 and a bandpass infrared filter104 are arranged on the optical axis of an optical system. An image ofinfrared light from the sample is formed on the light receiving surfaceof the MCT detector 102. The focal point of the optical system isadjusted by a focal point adjusting screw 106. The MCT detector 102inputs a detection signal to a preamplifier 107.

Example 6

An acrylonitrile (25%)-styrene (75%) copolymer (AS resin) was used asthe material of an invisible. barcode. This resin was pulverized to havean average size of 11 μm to prepare toner not containing pigments. Thistoner was used as toner of a laser beam printer to form an invisiblebarcode on label paper. The formed barcode corresponds to a barcode witha basic width of 300 μm printed on an existing article selected atrandom. The label paper on which the invisible barcode was formed waspasted on a plastic card to form a sample (A).

FIG. 9 shows the construction of a barcode reading apparatus used inthis example. FIG. 10 is a perspective view showing the positionalrelationship between a stage for holding a sample and a tubular halogenlamp with reflector in this barcode reading apparatus. FIG. 11 is asectional view of the tubular halogen lamp with reflector. The barcodereading apparatus of this example will be described in more detail belowwith reference to FIGS. 9 to 11.

Rollers 201 are placed below a frame 200, and a pulse stage 202 and astage top plate 203 are mounted on these rollers 201. A sample 1 onwhich the invisible barcode label paper is pasted is placed on the stagetop plate 203 and aligned with reference to a target 204. Thesecomponents are moved between a detection position (indicated by thesolid lines) and a heating position (indicated by the alternate long andshort dashed lines) along a direction L shown in FIG. 10 by an air valve(not shown). The pulse stage 202 moves in a direction S (barcode scandirection) shown in FIG. 10 under the control of a stage controller (notshown). A micrometer 205 adjusts the height of the stage top plate 203.

A tubular halogen lamp 206 with a reflector 207 is so installed as to bepositioned above the sample when the sample moves to the heatingposition. As shown in FIG. 11, the section of the reflector 207 forms apart of an ellipse. The halogen lamp 206 is arranged at the first focalpoint of the reflector 207 and installed in a lamp house. Light from thehalogen lamp 206 is linearly condensed to have a width D at a secondfocal point F2. The lamp house includes a mechanism (not shown) foradjusting its vertical position. To protect the sample from overheating,this mechanism adjusts its vertical position so that the upper surfaceof the sample deviates 2 to 6 mm from the focal point of the reflector207. A radiation thermometer (not shown) detects the temperature on theupper surface on the sample. If the temperature exceeds a set value, asafety device sends an alarm signal to the power supply of the halogenlamp 206 to cut off the switch.

An optical chopper unit is attached to the frame 200. In this unit, amotor 208 is mounted facing down, and an optical chopper 209 is held bythis motor 208 so as to be rotatable above the sample in the detectionposition. An optical sensor 210 measures the rotating speed of theoptical chopper 209. A frame 211 for safety is formed around the opticalchopper 209. A window is formed in this frame 211 to allow infraredemission from the sample to reach a detecting optical system. The frame200 also holds a lens barrel 101 of the detecting optical systemincluding a calcium fluoride lens 113, a bandpass infrared filter 104,and an MCT detector 102 above the optical chopper 209. The calciumfluoride lens 113 has a diameter of 25 mm and a focal length of 50 mm.The focal point of the optical system is adjusted by a focal pointadjusting screw 106. Although not shown, an optical stop is arranged onthe optical axis of the detecting optical system.

A barcode is read by using the above reading apparatus as follows.First, the pulse stage 202 is set in the detection position, and asample is placed on the stage top plate 203. The optical axis of thedetecting optical system and a barcode information portion of the sample1 are aligned with reference to the target 204. That is, the end of aleft margin of the barcode is positioned on the optical axis of thedetecting optical system. This position is an initial position of thesample 1. Next, the air valve is activated to move the sample 1 togetherwith the stage top plate 203 to the heating position. The halogen lamp206 heats the upper surface of the sample 1 to 75° C. in the initialposition. This heating excites. molecular vibration of a cyano groupcontained in the invisible barcode material on the sample 1, andinfrared emission near 4.5 μm occurs accordingly. After heating, thestage top plate 203 is immediately returned to the initial position, andthe infrared emission is detected in a cooling process as follows.

A stage controller (not shown) supplies a signal to scan the pulse stage202 at a fixed rate of. 20 mm/sec in the direction S. The infraredemission from the sample 1 is condensed by the calcium fluoride lens 113through the optical chopper 209 and detected by the MCT detector 102through the bandpass infrared filter 104. The calcium fluoride lens 113has a diameter of 25 mm and a focal length of 50 mm. The distancebetween the lens and the detector is 2.5 times (magnification is ×2.5)the distance between the lens and the sample. The MCT detector 102 has ahighest-sensitivity wavelength of 4.5 μm, and its light receivingsurface is composed of square elements of 1 mm side. When in use, thisMCT detector 102 is cooled to −200° C. by liquid nitrogen 102A. A biaspower supply (not shown) supplies power to the MCT detector 102. The MCTdetector 102 converts a change in its electrical resistance caused bythe infrared emission into a voltage and thereby generates a detectionsignal.

A preamplifier amplifies the output from the MCT detector 102 by 100times, and a lock-in amplifier. detects and amplifies the in-phasesignal. A digital sampling oscilloscbpe, (not shown) triggered by anoutput from the optical chopper 209 displays the waveform of thedetection signal. The detection signal is input to an A/D conversionboard of a computer and subjected to data processing as follows. Adecoder decodes the processed signal.

FIG. 12 shows data of a differential coefficient obtained by smoothingthe detection signal of the infrared emission from the sample anddifferentiating the smoothed signal as in Example 1. In this example,barcodes were read by using the JAN code rules. As described earlier, aJAN code of eight characters is composed of left guard bars “101”, fourdata characters on the left side, center bars “01010”, three datacharacters on the right side, one modular check character, and rightguard bars “101”. One character (number) is expressed by forming amodule seven times as large as the basic width by using two bars and twospaces.

In the differential coefficient data shown in FIG. 12, the left guardbars “101” are first detected. The center bars “01010” are then detectedby looking up the left guard bar detection signal. The left guard barsare removed from the measured signal. Next, the differential coefficientshown in FIG. 12 is divided into each characters within left to centerbars by two bars and two spaces. Meanwhile, reference signals of modulescorresponding to individual numbers are obtained. The correlationbetween each reference signal and an actually measured signal iscalculated to obtain a number by which the correlated value of the twosignals is a maximum. By this manipulation, numbers corresponding to themodules divided as above are determined.

In this example, when the upper surface of the sample was heated to 75°C., the barcode read accuracy was 90% or more. However, the readaccuracy was about 50% and about 90% when the upper surface of thesample was 50° C. and 90° C., respectively.

Instead of the sample (A), samples (B) and (C) were prepared by formingbarcodes as follows.

Sample (B); A.polyacrylonitrile powder was dispersed in a 5 wt % aqueouspolyvinyl alcohol solution at a ratio of 2 wt % with respect topolyvinyl alcohol. The resultant material was used to form a barcode onplain paper.

Sample (C): An acrylonitrile-styrene copolymer (AS resin) was.dissolvedin toluene, and the solution was mixed with wax. A film substrate wascoated with the resultant material and dried to form a heat-sensitiveink ribbon. This heat-sensitive ink ribbon was used to form a barcode onplain paper.

Results similar to those described above were obtained when thesesamples (B) and (C) were used.

Example 7

As shown in FIG. 13, a thermal head 216 was used as heating meansinstead of the tubular halogen lamp in Example 6. This thermal head 216has a mechanism for adjusting its vertical position. When a sample movesto the heating position, the thermal head 216 is moved downward andpushed against the sample. The pulse width of a pulse voltage to beapplied to the thermal head 216 is related to the temperature on theupper surface of the sample and the signal intensity of infraredemission. In this manner a pulse voltage with an appropriate pulse widthis applied to the thermal head 216.

As in Example 6, a pulse stage 202 is set in the detection position.After a sample 1 is placed on a stage top plate 203, an air valve isactivated to move the sample 1 together with the stage top plate 203 tothe heating position. The thermal head 216 heats the upper surface ofthe sample 1 to 75° C. in the initial position. After that, the stagetop plate 203 is immediately returned to the initial position, andinfrared emission is detected in the process of cooling as in Example 6.

The same sample (A) as used in Example 6 was used to detect infraredemission from a barcode following the same procedure as in Example 6.Data optimized in the same manner as in Example 6 was decoded by adecoder. Consequently, the decoded data matched the result of decodingof the original data printed on the existing article.

Example 8

When measurement is performed in the same manner as in Example 7, asignal of a peripheral portion is detected as follows in addition to asignal of a barcode information portion shown in FIG. 14. This signal ofthe peripheral portion is used as a background level to correct thedetection signal of the barcode information portion.

A pulse stage 202 is set in the detection position, and a sample 1 isplaced on a stage top plate 203. After the barcode information portionis aligned in this initial position, an air valve is activated to movethe sample 1 together with the stage top plate 203 to the heatingposition. A thermal head 216 heats the upper surface of the sample 1 to75° C. in the initial position. After that, the stage top plate 203 isimmediately returned to the initial position. In the subsequent coolingprocess, while the pulse stage 202 is moved at a fixed rate of 20mm/sec, infrared emission from the barcode information portion isdetected and input as a file to a computer.

Next, the stage top plate 203 is returned to the initial position, andthe optical axis of the detecting optical system is aligned with theperipheral portion of the barcode information portion. After that,infrared emission from the peripheral portion is detected and input as afile to the computer.

As data processing, smoothing, binarization by differentiation, andwidth adjustment were performed following the same procedures as inExample 1 for a difference obtained by subtracting the peripheralportion signal from the barcode information portion signal. Dataoptimized in the same manner as in Example 1 was decoded by a decoder,and the decoded data matched the result of decoding of the original dataprinted on the existing article.

Example 9

When measurement is performed in the same manner as in Example 7, asignal of a peripheral portion is detected as follows in addition to asignal of a barcode information portion shown in FIG. 14. This signal ofthe peripheral portion is used as a background level to correct thedetection signal of the barcode information portion.

A pulse stage 202 is set in the detection position, and a sample 1 isplaced on a stage top plate 203. After the barcode information portionis aligned in this initial position, an air valve is activated to movethe sample 1 together with the stage top plate 203 to the heatingposition. A thermal head 216 heats the upper surface of the sample 1 to75° C. in the initial position. After that, the stage top plate 203 isimmediately returned to the initial position. In the subsequent coolingprocess, the pulse stage 202 is scanned in a direction S at a fixed rateof 20 mm/sec. Synchronizing with this scanning, rollers 201 are rotatedto reciprocate the sample 1 in a direction L at a rate of 200 mm/secsuch that the barcode information portion and the peripheral portion arealternately scanned across the edge (the boundary between theinformation portion and the peripheral portion) of the barcodeinformation portion shown in FIG. 14. In this manner, infrared emissionfrom the barcode information portion and that from the peripheralportion are detected. The obtained signals are amplified by an ACcoupled-amplifier and input as a file to a computer.

Smoothing, binarization by differentiation, and width adjustment wereperformed for the obtained data. When the optimized data was decoded bya decoder, the decoded data matched the result of decoding of theoriginal data printed on the existing article.

Example 10

FIG. 15 is a block diagram showing a barcode reading apparatus used inthis example. This apparatus uses a silicon macro camera lens as anoptical element for condensing and guiding infrared emission from asample to a detector, and a PtSi FPA-CCD as an infrared detector.

As in the apparatus shown in FIG. 9, a pulse stage and a stage top plate203 are mounted on rollers. A sample 1 is placed on the stage top plate203 and. aligned by looking up a target 204. When the sample 1 moves tothe heating position, a tubular halogen lamp 206 with reflector heatsthe sample 1. In the detection position, a silicon macro camera lens301, a bandpass infrared filter 302, and a PtSi FPA-CCD 303 arepositioned above the sample 1. The silicon macro camera lens 301includes an antireflection coating and has a viewing angle of 20°×15°and a minimum focal length of 250 mm. The FPA-CCD 303 has ahighest-sensitivity wavelength of 4.5 μm, includes 320×240 pixels, andis cooled to −200° C. by a Stirling cooler 304 when in use. Electriccharge stored in the EPA-CCD 303 is transferred to a reader 305synchronizing with a sync signal. The signal transferred to the reader305 is amplified by a preamplifier 306, input to an A/D conversion board308 of a computer 307, and decoded by a decoder 309.

The pulse stage is set in the detection position, and the sample 1 isplaced on the stage top plate 203. The center of the FPA-CCD 303 and thecenter of a barcode information portion of the sample 1 are aligned withreference to the target 204. An air valve is activated to move thesample 1 together with the stage top plate 203 to the heating position.The halogen lamp 206 heats the upper surface of the sample 1 to 75° C.in the initial position. After heating, the stage top plate 203 isimmediately returned to the initial position, and infrared emission isdetected in the cooling process.

The infrared emission from the sample 1 was input as a two-dimensionalimage file to the computer 307. Smoothing and signal integration withina set.range were performed for this two-dimensional measured data toobtain one-dimensional data. this one-dimensional data was subjected tobinarization by differentiation and width adjustment. The processed datawas decoded by the decoder 309, and the decoded data matched the resultof decoding of the original data printed on the existing article.

Example 11

Toner made from the same AS resin as used in Example 6 was used as tonerof a laser beam printer to form an invisible barcode on plain paper.This barcode corresponds to a standard-size barcode with a basic widthof 300 μm printed on an existing article selected at random.

When the barcode is read with spatial resolution higher than the basicwidth of its module, a signal having a time width corresponding to thewidth of a bar or a space is obtained (FIG. 16A). When the barcode isread with spatial resolution equivalent to the basic width of itsmodule, the read time can be shortened, but the apparatus function issuperposed (convoluted) on the amplitude of a signal (FIG. 16B).Consequently, as can be seen by the comparison of the signal shown inFIG. 16B with the signal shown in FIG. 16A, peaks of the signal with lowintensity become low, and this obscures the correspondence between thetime width of the signal and the width of a bar or a space. Therefore,it is desirable to allow a filter that serves as a function of thefrequency of a detection signal as shown in FIG. 16B to act on thesignal to remove (deconvolute) the apparatus function and correct theamplitude and then binarize the signal.

In this example, an invisible symbol reading apparatus as shown in FIG.6 was used, the temperature was controlled by turning on and off atemperature controller, and the barcode was read in the cooling processas follows. A sample 1 was held on a holder 32 by a plate-like magnetand so adjusted that a barcode information portion of the sample 1 waspositioned at the focal point of a detecting optical system by lookingup a target 34. To adjust the optical axis, guide light with awavelength of 640 nm was irradiated from a semiconductor laser (notshown). However, the focal point of this guide light shifted from thefocal point of infrared emission. Hence, after the in-plane position ofthe optical axis was adjusted by the guide light, final adjustment wasperformed by monitoring an output from an MCT detector 17 on anoscilloscope. The temperature to which the sample 1 was heated under thecontrol of a temperature controller 33 was adjusted to 90° C. A signalfrom a built-in thermocouple (not shown) of the holder 32 was input tothe temperature controller 33. On the-basis of this signal, the currentto be supplied to a bar heater 31 was adjusted. Before the sample 1 wasscanned, the temperature controller 33 was turned off to put the sample1 in the cooling process. While a pulse stage 11 was moved at a fixedspeed by a signal from a stage controller 12, infrared emission at thefocal point was detected. The start and end positions of the barcodewere measured by taking margins for these positions by using limitswitch signals from the pulse stage 11. Data of the detection signal wasinput as a file to a computer 21.

If the signal intensity is high, an optical stop can be installed on theoptical axis to reduce wavelength components except for the wavelengthto be measured, thereby effectively performing wavelength selection. Ifthis is the case, a bandpass infrared filter 16 is unnecessary.

Filtering was performed for actually measured data as shown in FIG. 16Bto remove the apparatus function and smooth the data. After that,binarization by differentiation and width adjustment were performed inthe same manner as in Example 1. When the data was decoded by a decoder,the decoded data matched the result of decoding of the original dataprinted on the existing article.

Example 12

A heat sensitive ink ribbon similar to that used in the formation of thesample (C) in Example 6 was used to form an invisible barcode on aplastic card. This barcode corresponds to a standard-size barcode with abasic width of 300 μm printed on an existing article selected at random.

In this example, a barcode reading apparatus having heating means and adetecting optical system shown in FIG. 17 was used. This readingapparatus has a thermal bar 51 separated from a lens barrel 101 of thedetecting optical system including a Cassegrain lens and the like. Aguide 52 is formed below these members. A card type sample 1 is placedwith its barcode printed surface facing up on the guide 52 having acurved portion, and is conveyed by rotation of a roller 53 An opticalsensor 54 installed before the heating position senses the approach ofthe sample 1, and the temperature of the thermal bar 51 is raised by thesensor signal. The thermal bar 51 is usually operated by remaining heatin order to prevent thermal deterioration of the opposing roller 53. Thesample 1 is pushed and heated by the thermal bar 51 while passingthrough the gap between the curved portion of the guide 52 and thethermal bar 51. A thermocouple senses the temperature of the backsurface of the sample 1, and a temperature controller controls theheating temperature. The sample 1 whose barcode surface is heated by thecontact with the thermal bar 51 is conveyed toward the infrared emissionsensing position by the roller 53. An optical sensor 55 installed beforethe sensing position senses the passage of the sample 1 and supplies ameasurement start signal to a computer. Infrared emission is detected inthe cooling process. This infrared emission from the barcode on thesample 1 is guided into the lens barrel of the detecting optical system.The infrared emission intensity as a function of time (convertible intoa function of position because the roller rotates at a fixed rate) isinput as a file to the computer.

Following the same procedures as in Example 1, smoothing, binarizationby differentiation, and width adjustment were performed for the data.The data optimized in the same manner as in Example 1 was decoded by adecoder. Consequently, the decoded data matched the result of decodingof the original data printed on the existing article.

In the apparatus shown in FIG. 17, the rotating speed of the roller 53can be so changed as to increase the conveyance speed of the sample 1when the thermal bar 51 heats the sample 1 and decrease the conveyancespeed of the sample 1 when the optical system detects infrared emission.

Also, as in an apparatus shown in FIG. 18, a plurality of rollers 61,62, 63, 64 a, 64 b, 65, 66, 67 a, and 67 b can be used. In thisapparatus, the rotating speeds of these rollers are changed to increasethe conveyance speed of the sample 1 when the thermal bar 51 heats thesample 1 and decrease the conveyance speed of the sample 1 when theoptical system detects infrared emission.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for reading an invisible symbolcomprising the steps of: exciting molecular vibration of an invisiblesymbol formed on a sample and containing a material which emits infraredlight when heated; detecting infrared light emitted from the invisiblesymbol; providing a differential coefficient of a detection signalcorresponding to a position on the sample; determining, based onthreshold values set for the differential coefficient, alternatingmaximum and minimum values of differential coefficients outside saidthreshold values; and binarizing the detection signal by using themaximum or minimum value as a leading or trailing edge of a binaryfunction.
 2. The method according to claim 1, wherein the step ofdetecting infrared light emitted from the invisible symbol is performedin a process of cooling the sample.
 3. The method according to claim 1,wherein the invisible symbol is a linear barcode, a basic width of thelinear barcode is calculated from the binary function, and the binaryfunction is corrected to an integral multiple of the basic width.
 4. Themethod according to claim 1, wherein the invisible symbol is a linearbarcode, a basic width of the linear barcode is calculated by detectinga reference code pattern, and a data character is read on the basis ofthe calculated basic width.
 5. The method according to claim 1, whereina signal level of an underlying substrate is used as a signal level ofbackground to correct a signal level of the invisible symbol.
 6. Themethod according to claim 1, wherein the invisible symbol is made from apolymer containing a cyano group.
 7. The method according to claim 1,wherein the sample is heated to 60 to 100° C.
 8. The method according toclaim 1, wherein the molecular vibration of the invisible symbol isexcited by heating.
 9. The method according to claim 1, wherein themolecular vibration of the invisible symbol is excited by lightirradiation.
 10. An apparatus for reading an invisible symbolcomprising: a unit configured to excite molecular vibration of amaterial of the invisible symbol, the invisible symbol formed of asample and containing a material which emits infrared light when heated;a detector configured to detect infrared light emitted from theinvisible symbol; and an arithmetic operation unit configured tobinarize a detection signal from said detector; wherein said arithmeticoperation unit calculates differential coefficients of the detectionsignal corresponding to positions on the sample, determines, based onthreshold values set for the differential coefficients, alternatingmaximum and minimum values of the differential coefficient in regionsoutside the threshold values, and binarizes the detection signal byusing the maximum or minimum value as a leading or trailing edge of abinary function.
 11. The apparatus according to claim 10, wherein saidheater heats the sample is installed in a position apart from saiddetector, and said apparatus further comprises a conveyor configured tomove the sample from a heating position of said heater to a detectionposition of said detector.
 12. The apparatus according to claim 10,further comprising a controller configured to turn off said heaterbefore detection by said detector.
 13. The apparatus according to claim10, further comprising: an optical modulator configured to opticallymodulate the infrared light emitted from the invisible symbol; and saiddetector comprising a phase detector configured to detect a phase of adetection signal.
 14. The apparatus according to claim 10, furthercomprising a bandpass infrared filter for transmitting infrared light ina specific wavelength region of the infrared light emitted from theinvisible symbol.
 15. The apparatus according to claim 13, wherein saidbandpass infrared filter transmits infrared light near 4.5 μm peculiarto a cyano group.
 16. The apparatus according to claim 10, furthercomprising a calcium fluoride lens.
 17. The apparatus according to claim10, further comprising a Cassegrain lens.
 18. The apparatus according toclaim 10, wherein said detector is a mercury cadmium tellurium detector.19. The apparatus according to claim 10, wherein said detector forms afocal plane array.
 20. The apparatus according to claim 19, wherein anelement constructing said focal plane array is made of a materialselected from the group consisting of platinum silicide and indiumantimony.
 21. The apparatus according to claim 10, wherein the unitconfigured to excite molecular vibration of a material of the invisiblesymbol is a heater.
 22. The apparatus according to claim 10, wherein theunit configured to excite molecular vibration of a material of theinvisible symbol is a light source.